Q. I need to use the username Administrator in my Azure VM but it won't let me, what can I do?
A. The Azure Resource Manager will block any attempt to use the username Administrator when provisioning a VM and this is for good reason. It's a bad idea to leave a privileged account with the default name as this leaves them susceptible to attack. If you have an application that traditionally needs to use a named Administrator account that is not a good thing and it would be better to be able to configure a custom account for the application that only has the permissions actually required rather than an account with full administrative privileges which is typically the lazy option that leaves it susceptible to attack and abuse.
If there is a legitimate reason you must have an account named Administrator in an Azure VM you can add the account within the VM post creation and it will function as normal. It is only during creation the use of Administrator as the account name is blocked.